Standard methods of accomplishing pitch change in propellers, helicopter rotors, or some fans with limited number of blades, usually include a root attachment mechanism such as a ball/roller bearing and/or flex member, which allows rotation of the blade with relatively low friction. Such devices can be heavy due to the high blade centrifugal forces they must support. A further complication is that centrifugal loads on blade plate-like structures also produce significant twisting forces that pitch control systems must overcome. These forces tend to rotate the blade towards a dangerous flat pitch position, such that a malfunction in pitch control could result in rotor overspeed(s) and potential blade loss.
The force required to change the pitch angle of a rotating blade can be appreciable. In propeller technology, where variable-pitch was incorporated many years ago, this force is usually referred to as the Total Twisting Moment (TTM), which is the net sum of three basic forces. The first is Centrifugal Twisting Moment (CTM) that originates from the non-symmetrical mass distribution (i.e. oblong airfoil, non-circular) of a blade's airfoil about its pitch change axis. Second is Aerodynamic Twisting Moment (ATM) caused when the effective center of pressure on each airfoil section is aligned forward or aft of the pitch change axis and that airload causes a twisting load about the blade pitch axis. Last is Frictional Twisting Moment (FTM) which resists motion and develops in the retention bearings that support the blade, due to high centrifugal loads acting on them. Among these, CTM is by far the greatest, with ATM and FTM distant seconds. CTM acts to rotate a blade toward low pitch. Because the aerodynamic center of pressure of a blade's airfoil is usually forward of its pitch change axis, ATM normally acts to increase blade pitch, opposing CTM. FTM caused by friction, acts to oppose blade pitch change in either direction.
With TTM being dominated by CTM, the pitch control system exerts a twisting load in the direction of increased pitch to hold blade pitch constant, and a higher force yet to overcome FTM in order to increase blade pitch. If there is a malfunction and/or loss of control of the pitch system, a blade will naturally turn toward low pitch. Because low blade pitch results in less rotational resistance for the engine, the situation can result in a dangerous overspeed of the rotor and engine with a powered engine. Loss of engine power is usually accompanied by loss of pitch control. Again TTM can turn the blades to low pitch, but rotor thrust suddenly switches to a high drag force that can cause possible loss of aircraft control and/or result in rotor overspeed. Rotor overspeed is more likely if the rotor is driven by a turbine engine rather than a piston engine, especially if the former has a “free” turbine that powers the rotor. In a turbofan engine, with the great number of blades in the fan, loss of pitch control and the turning of blades to low pitch could cause significant drag and overspeed conditions. Therefore, backup pitch-change systems, pitch safety latches, or other complicated, expensive and/or heavy solutions have not been attractive to date.
To prevent undesirable change in pitch tendencies, the conventional solution is to add a counter-weight to the side of a blade at/near its root end. This weight must be of sufficient mass and position to create a net TTM that will always be able to overcome all inherent blade twist loads and drive the blade towards high pitch, or at least hold pitch setting to prevent movement toward low pitch. The counter-weight mass on each blade is normally quite substantial and adds undesirable weight to the rotor, as well as additional load to the rotor hub and blade retention bearings. Also, there is the added risk of failure of a counter-weight support arm, possible impact damage if the weight strikes the aircraft fuselage, combined with dangerous unbalance of the rotor.
Another means of addressing undesirable change in pitch tendencies involves use of a back-up pitch change system such as an auxiliary electric pump to backup a hydraulic system, which could drive pitch angle high to a “Feather” position, etc. Also, a high friction device(s), such as a linear ACME thread in the pitch actuation system, or a harmonic drive device, or latching device could be utilized, any of which can be designed to hold the current pitch position if pitch control is lost. The latter have the beneficial effect of allowing continued operation if the malfunction occurs in a fortuitous operating condition and/or there is continuous power from the engine. However, if pitch is held in a less-than-optimal position for gliding, or twin engine operation, there could still be a compromised flight control situation (s) that develops from the increased drag forces generated.
Turbofan engines, which derive 90 percent of their thrust from the large fan up front (essentially a ducted propeller), still do not make use of variable pitch today. Because engine speed generally remains nearly constant with jet engines, power is adjusted with fuel flow. However, all of today's jet engines have fixed pitch blades, which utilize larger, compromised airfoils to provide sufficient thrust that enables takeoff capability at full load on a hot day. Consequently, these engines suffer an efficiency penalty in cruise, when there is more blade airfoil than required, as it is at less than optimum blade angle. If aircraft travel is to survive for future generations, efficiency and fuel savings will need to be significantly increased.